Salt and solute rejecting membranes suitable for use in the desalination of aqueous solutions are the subject of numerous patents. Cadotte (U.S. Pat. No. 4,259,183) discloses permselective (salt rejecting) multilayer membranes in which there is a microporous polysulfone substrate layer and a juxtaposed polyamide layer made from an aromatic triacyl-halide and an essentially monomeric, polyfunctional secondary amine. Scala et al (U.S. Pat. No. 3,744,642) discloses multilayer membranes in which one layer is a porous substrate and the other juxtaposed layer is a polyamide, a polyphenyl ester, or a polysulfonamide. Hara et al (U.S. Pat. No. 4,353,802) discloses semipermeable composite membranes in which the membrane material is crosslinked using polyfunctional aromatic acid halides. Kamiyama et.al. (U.S. Pat. No. 4,619,767) discloses permselective multilayer membranes having (1) a microporous substrate layer (2) an ultra thin layer of a crosslinked polyvinyl alcohol/amino compound and (3) a porous inner layer of water insoluble polyvinyl alcohol located between the porous substrate and the ultrathin layer. Other patents disclosing the preparation and properties of thin film composite membranes are U.S. Pat. Nos. 3,951,815; 4,005,012; 4,039,440; 4,277,344; 4,885,091; 4,895,661; 4,802,984, 4,913,816 and many others.
A discussion of chlorine tolerance of reverse osmosis membranes, (equally applicable to nanofiltration membranes be they anisotropic or thin film composite) is presented by Robert J. Petersen in a 1986 paper entitled "The Expanding Roster of Commercial Reverse Osmosis Membranes". It was stated therein that the thin film composite membranes in which the thin polyamide layer was composed of the reaction product of trimesoyl chloride and piperazine had the best chlorine tolerance for a polyamide TFC membrane, but might not be fully resistant to chlorine degradation despite these claims. Time has proven these claims of chlorine resistance to be invalid.
A polymeric material having a large surplus of primary, secondary, and even tertiary amine functionalities will have a large chlorine tolerance to hypochlorite or chlorine containing solutions in the 0.1 to 1.0 ppm concentration range. The thin film composite membranes of the present invention have intentionally been constructed with large excesses of amine functional groups for the express purpose of upgrading their abilities to neutralize and tolerate small quantities of chlorine or hypochlorites which may be present in water to which these TFC membranes may be exposed. Concurrently, the membranes of the present invention are also protected against chlorine destruction by the co-addition of double bond containing water miscible latexes. These become a mixed component of the TFC during the formation of the thin film polyamides at the interface of a hexane solution containing acyl chlorides and the aqueous phase containing piperazine, polyamines, a butadiene-styrene latex and caustic to accelerate the formation of the interfacial polyamides. The unsaturated latex (the subject of W. A. Waite's U.S. Pat. No. 4,913,816) reacts with chlorine present in feed waters to also render the TFC membrane chlorine tolerant.
The nanofiltration membrane of the present invention will contain two species capable of making the TFC membrane chlorine (or hypochlorite) tolerant. These species are:
a.) An added precipitated latex such as butadiene-styrene as disclosed in U.S. Pat. No. 4,913,816 which patent is incorporated herein by reference.
b.) Polyamines incorporated into the TFC polyamide ##STR1##
The addition of polyamides to the composition of the amino portion of the TFC polyamides of the present invention have been found to do much more than provide a reducing compound for the neutralization of excess chlorine normally added to the feed waters to be treated.
The polyamines used in preparing the TFC polyamine-polyamide of this invention also provide excess crosslinking capability during the formation of the TFC polyamide. Thus excess crosslinkage, when reacted with diacyl or triacyl halides to form a polyamine-polyamide thin film has been found to effect the degree separation of sodium and alkaline earth salts such as Ca.sup.+2 and Mg.sup.+2. One such example of a polyamine is tetraethylene pentamine (TEPA): EQU NH.sub.2 --CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CH.sub.2 --NH.sub.2
which has no less than five reactive sites for combination with a triacyl halide, such as trimesoyl chloride ##STR2## or a diacyl halide, such as isophthaloyl chloride below: ##STR3## The prolonged reaction of polyamines with di or triacyl halides will result in a TFC interfacially created polymer having a high degree of crosslinkage, which should result in a composite polymer having minimum hydrogen bonding between the polymer chains due to spacial restrictions (or constrictions) of the network polymer formation. It is believed that such special constrictions will allow the formation of pore sizes of higher volume and average diameter than those formed by the direct reaction of di or triacyl halides with a bifunctional amine especially in view of the fact that using the trifunctional trimesoyl chloride sometimes forms lightly crosslinked amides due to the formation of a COOH group rather than a network polymer as shown below: ##STR4## where n is customarily &gt;p.